The accuracy of out of field dose calculations in commercial treatment planning system using GATE/GEANT4 Monte Carlo simulation

The out-of-field dose has particular importance because of the increased risk of radiation-induced second cancer and late effects due to low doses for out-of-field organs at risk (OARs). Although the out-of-field doses are clinically related to the doses to OARs, they receive less attention and are not well documented because of the limitations in the treatment planning system (TPS) in calculating them. Today, Monte Carlo (MC) is considered the most accurate and alternate method in calculation of the dose distribution. This study is evaluated the ac-curacy of the out-of-field dose calculated by the commercial PROWESS (c) V.5.01 TPS. A comparison for out-of -field dose profiles (DPs) between TPS and MC for field sizes from 10 x 10 cm 2 up to 20 x 20 cm 2 with an increment of 5 cm at depths 1.5, 5, and 10 cm. Out-of-field DPs were calculated for each field size and each depth from the center of the treatment field with an increment of 2 mm along the in-plane direction to a certain distance at which the TPS reported zero doses. Each curve was normalized to the calculated dose at CAX. The relation for the dependence of the out-of-field doses on the field size and depth in the phantom was also eval-uated. The comparison between TPS and MC showed that the PROWESS (c) V.5.01 TPS underestimated the out-of -field dose compared with MC simulation. The degree of the underestimation of dose increases as the distance from the field edge increases. The out-of-field dose depends on field size and depth.Limitations of PROWESS TPS to detect the out-of-field doses could be lead to inaccuracies of underestimation of secondary cancer induction risk associated with these doses. The quantifying of the accuracy of the out-of-field dose calculations by Prowess TPS resulted in underestimated for the out-of-field doses compared with Gate MC simulation in the range from 34% to 65% at different field sizes and depths. The underestimation of the out-of -field dose increased to 65% as the field size and depth increased.